In the leading-edge technologies of recent precision fine die and mold machining, sub-micron machining accuracy has been achieved by using highly accurate machining centers. For the purpose of enhancement of machining accuracy and productivity efficiency, such a technique has been required that measures a tedious and time-consuming tool setting (a gap between a tip end of a tool that is rotating at a high speed and a workpiece) with sub-micron accuracy. Currently, fine machining is carried out by using a small-diameter tool of a machine tool such as a machining center with high accuracy. For the enhancement of the machining accuracy, machining accuracy and machining efficiency may be significantly affected by not only precision of a machine tool but also information regarding a cut-lip end (cutting edge) of the tool on the machine that is important information for determining a position of the tool relative to the workpiece and its in-feed rate. Unfortunately, measurement accuracy handled in an optical non-contact and on-machine tool measurement method that is commercially available is approximately several microns due to the diffraction phenomenon of an emitted light, and this is insufficient for tool measurement with sub-micron measurement accuracy.
In conventional on-machine tool measurement methods, a tool is irradiated with a laser beam, and a light diffracted from the tool is detected, so as to measure a diameter and a cut-lip shape of the tool. If the tool is rotating at a high speed, the frame rate of a camera for acquiring the diffracted light cannot catch up with the rotational speed, which makes the measurement difficult (see Patent Documents 1, 3). Conventional methods using the diffracted light require a reference knife-edge, so that it is necessary to find a relative distance between the reference knife-edge and a workpiece with high accuracy (see Patent Documents 2, 4).
FIG. 9 is a drawing explaining a measurement principle of the optical diffraction method (see Patent Documents 2, 4). As shown in the drawings, a light diffraction phenomenon occurs if a fine gap x between a small-diameter tool that is a measurement target and a reference knife-edge is irradiated with a line laser beam. The diffracted light is concentrated by a Fourier transformer lens, and at the same time, a diffraction pattern is acquired on a focal plane located at a focal distance f by using a camera or the like. Peaks w−1, w+1 are then detected that are first-order diffracted lights, a distance between the peaks is measured as w, and the fine gap x between the cut-lip of the small-diameter tool and the reference knife-edge is measured using the formula (1) and a known wavelength λ.
                              [                      Formula            ⁢                                                                      ⁢                                                                    (            1            )                    ]                ⁢                                  ⁢                  x          =                      1.4303            ⁢            λ            ⁢                                          1                +                                                      (                                                                  2                        ⁢                        f                                            W                                        )                                    2                                                                                        (        1        )            